1. Technical Field
The present invention generally relates to alkali metal thermal electric conversion (AMTEC) cells and more particularly to a thermal transfer apparatus for controlling the temperature of the working fluid, solid electrolyte structure and evaporation surface within the cell.
2. Discussion
AMTEC cells generally comprise a closed chamber separated into a high pressure zone and a low pressure zone by a solid electrolyte structure (SES). An alkali metal working fluid, such as sodium, is present in the high pressure zone. The SES is operable for conducting alkali metal ions but not neutral alkali metal atoms.
During operation of a vapor-vapor type of such a cell, a heat source raises the temperature of the working fluid within the high pressure zone to a high temperature and a corresponding high vapor pressure. The high pressure creates a vapor pressure differential across the SES. In response to this pressure differential, the neutral alkali metal atoms give up electrons to a permeable electrode in contact with the SES. The resulting ions travel through the SES crystal lattice with a preferred direction due to the pressure gradient. Furthermore, the electrons are given up to power an external load.
The alkali metal ions exiting the SES are neutralized by electrons delivered from a permeable, electrically conducting electrode in contact with the SES on the low pressure side. The neutralized atoms migrate through the low pressure zone to a much cooler surface where the alkali metal condenses.
The condensed liquid alkali metal is then returned back to the higher pressure zone by means of a return line. The returned liquid alkali metal is evaporated at an evaporation surface at the hot end of the return line. The evaporated, high pressure alkali metal vapor is then conveyed to the high pressure side of the SES through a common plenum at the hot end of the cell to complete a closed cycle.
In AMTEC cells capable of achieving high conversion performance, the liquid metal returning from the condenser via the return line must be vaporized. The thermal energy required to vaporize the returning liquid is a substantial portion of the total energy required for cell operation. Although prior art attempts to add heat to this site, as well as to the SES and working fluid, have been generally successful, there is room for improvement in the art.
For instance, it would be desirable to provide means for optimally transferring heat to the evaporation surface, SES and working fluid so that thermal energy is more efficiently converted to electrical energy. More particularly, it would be desirable to provide an AMTEC cell capable of providing heat from the heat input surface of the cell to the evaporation surface of the working fluid such that entropy generation is minimized. Additionally, it would be desirable to provide an AMTEC cell imparting sufficient thermal energy to the working fluid after evaporation so that the thermal dynamic state of the working fluid along its flow path from the evaporator to the SES inner current collector is such that the working fluid does not condense anywhere along this path, especially on the inner current collectors of the SES. Furthermore, it would be desirable to provide an AMTEC cell capable of providing heat to the SES through conduction and radiation heat transfer from the heat input surface of the cell and other cell components and convective heat transfer from the working fluid.